Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD OF FORMING SOLDER FILM
FIELD OF THE INVENTION
The present invention relates to a method of forming
a solder film. More particularly, this invention relates to
a method suitable for forming on a printed circuit board a
solder circuit which is a solder layer formed beforehand on a
pad of a metallic circuit of the printed circuit board for
easy mounting of electronic parts thereon, or for forming of
a solder layer on an electronic part.
BACKGROUND OF THE INVENTION
In recent years, printed circuit boards (also
referred to as printed boards) were developed which comprise
an insulating substrate, such as a plastic substrate
(including film), a ceramic substrate, or a metal substrate
coated with a plastic or other substance, and an electronic
circuit formed thereon; and a technlque of soldering an
electronic part such as an IC element, semiconductor chip,
resistor, or capacitor to the circuit surface of such a
printed board to constitute an electronic device is being
widely employed.
In producing the above-described circuit device
having electronic parts mounted thereon, the ordinary method
for soldering a lead terminal of the electronic part to a
predetermined pad of the circuit comprises forming a thin
solder layer beforehand on either or both of the pad and the
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lead terminal, positioning the electronic part, and then
melting (reflowing) the thin solder layer(s) to solder the
electronic part on the circuit.
For the formation of the solder circuit (thin solder
layer), such methods as plating, dipping (immersion) in a
solder bath, and printing of a solder powder paste have been
employed. However, as the trend toward the increase in
mounting density, solder circuits have been required to have
even finer patterns and there also are desires for
improvements in working efficiency and on-specification rate
and for circuit pattern miniaturization. It is, therefore,
becoming difficult to cope with these requirements with the
above methods.
Among those'conventional methods, the plating method
is applicable for formation of solder circuits having highly
precise and fine patterns.
The plating method is classified into electroplating
and electroless plating. Use of electroplating, however,
encounters difficulties in attaining electrical conductivity
because, in actual printed circuit boards, the parts in which
solder circuits are to be formed are present independently
from the circuit parts in most cases. On the other hand,
electroless plating has a technical problem that it is
difficult to obtain a thick solder layer having a thickness
necessary in practical use, although the problem concerning
electrical conductivity in the electroplating is overcome.
2 1 0 9 4 6 2
A method of electrostatically applying flux-coated
solder powder particles on a circuit part has been proposed in
JP-A-3-50853, published March 5, 1991. (The term "JP-A" as
used herein means an "unexamined published Japanese patent
application".) However, this method is still unable to easily
produce a high-accuracy fine pattern.
Another method for the solder circuit formation has
been proposed in JP-A-4-10694, published January 14, 1992,
which comprises applying a flux on a circuit part by printing,
adhering a powdered solder to the flux-printed part, melting
the solder by heating it to a temperature not lower than the
melting point of the solder, and then blowing a gas on the
solder melt to level it to thereby form a solder circuit. This
method is disadvantageous in that a high degree of skill is
required because high-precision printing of a flux on a pad is
difficult and in addition there is a fear of bridging between
patterns spaced at a minute gap during the leveling of the
solder melt.
Also in the case where a solder coat (solder layer) is
formed on a lead terminal of an electronic part, there are the
same technical problems as those in the formation of solder
circuits.
As a result of extensive studies made in order to
improve the precision of solder patterns which are required to
be fine, it has been found that the plating method, which has
attained the highest precision, still has several
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2 1 0 9 4 6 2
problems and has to be improved in working efficiency and
other respects.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a
method of efficiently forming a precise, fine, bridge-free
solder film pattern on a metallic circuit or on an exposed
metal of an electronic part by a simple procedure without the
necessity of troublesome operations such as positioning.
It has been found that a certain chelate compound
acts on an exposed metallic surface to render it tacky, which
makes it possible to precisely adhere a powdered solder only
to the resulting tacky surface. The present invention has
been attained by the above discovery. That is, the present
invention is a method for forming a solder film, which
comprises selectively imparting tackiness to only an exposed
metallic part of a printed circuit board or electronic part
by means of a tacky layer-forming solution containing at
least one compound (tackiness-imparting compound) selected
from benzotriazole derivatives, naphthotriazole derivatives,
imidazole derivatives, benzoimidazole derivatives, mercapto-
benzothiazole derivatives, benzothiazole thiofatty acid
derivatives, and triazine derivatives, adhering a powdered
solder to the resulting tacky part, and then melting the
solder by heating to form a solder film on the part.
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In one aspect, the present invention provides a
method of forming a solder film which comprises selectively
imparting tackiness to only a predeteremined part of a
metallic surface of an article by means of a tacky layer-
forming solution containing at least one tackiness-imparting
compound selected from the group comprising organic chelate
compounds and salts thereof, adhering a powdered solder to the
resulting tacky part, and then melting the solder by heating
to thereby form a solder film.
BRIEF DESCRIPTION OF THE DRAWINGS
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Fig. 1 is a diagrammatic view illustrating a solder-
adhering technique in which a printed circuit board is placed
on a solder powder layer and a solder powder is allowed to
adhere to the printed board while a solder powder is being
fed.
Fig. 2 is a diagrammatic view illustrating a solder-
adhering technique in which a solder powder is allowed to
adhere while a solder powder layer is being moved.
Fig. 3 is an enlarged view of a through-hole part to
which a solder powder is adhered.
Fig. 4 is a diagrammatic view illustrating a solder-
adhering technique in which a printed circuit board is
thrusted into a solder powder layer which is being shaked by
a vibrator.
DETAILED DESCRIPTION OF THE INVENTION
Examples of printed circuit boards to which the
present invention can be applied include a single-sided
printed circuit board, a double-sided printed circuit board,
a multilayered printed circuit board, or a flexible printed
circuit board; each comprising a substrate obtained by
laminating a metal plate to, e.g., a plastic base, plastic
film base, glass cloth base, paper-based epoxy resin base or
ceramic base, or an insulated substrate obtained by covering
a metal base with, e.g., a plastic or ceramic, and a circuit
formed thereon. Further, the printed circuit board may be
one comprising a substrate in continuous sheet form and
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plurality of circuits independently formed thereon. Examples
thereof include a circuit board obtained by laminating a foil
of highly conductive metal such as copper to a thermoplastic
resin film, e.g., a polyimide or polyester film, with an
adhesive and further laminating a photosensitive resin film
as a resist, followed by light exposure, development and
etching to form plurality of circuits, and a circuit board
obtained by directly printing plurality of circuits on a
thermoplastic film.
Metal for use in circuit formation is copper in most
cases. Although copper is the most suitable metallic
material for the tackiness-imparting compound to be used in
this invention, i.e., benzotriazole derivatives, naphtho-
triazole derivatives, imidazole derivatives, benzoimidazole
derivatives, mercaptobenzothiazole derivatives, benzothiazole
thiofatty acid derivatives, triazine derivatives, etc., the
circuit-forming metal is not limited to copper and may be
other metals, such as gold, silver, tin, nickel, tungsten,
lead, copper alloys (e.g., Cu-PJ and solder. The metals
other than copper tend to show poor bondability with the
tackiness-imparting compound, as compared to copper.
The method of the present invention is also
applicable to ordinary electronic parts required to be
solder-coated. Examples of such electronic parts include
various kinds of packages such as a DIP (dual inline
package), an SIP (single inline package), a PGA (pin grid
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array), an SOP (small online package), A QFP (quad flat
package), and a TAB (tape-automated package).
Metal constituting a lead frame of such an electronic
part is, in most cases, Alloy 42 (40% nickel by weight and 60
iron by weight) or tin-containing copper. Although these
metallic materials like copper, are suitable for the tackiness-
imparting compound to be used in this invention, the metal
constituting the lead frame is not limited to these materials
and ma-y be others, such as copper and copper alloys (e.g., Ni-
Cu). Further, the part to be coated with a solder may be all
of the metallic part or maybe a part thereof.
Preferred examples of the tackiness-imparting compound
to be used in the present invention, which acts on a metal to
develop tackiness, include benzotriazole derivatives
represented by formula (1), naphthotriazole derivatives
represented by formula (2), imidazole derivatives represented
by formula (3), benzoimidazole derivatives represented by
formula (4), mercaptobenzothiazole derivatives represented by
formula (5), benzothiazole thiofatty acid derivatives
represented by formula (6), and triazine derivatives
represented by formula (7):
~ - N
P~
(wherein Rl represents hydrogen atom or an alkyl group)
~~;B
2~ 09462
~ ~ / ~ ~ N (2)
R 2 ~ J
R 3
(wherein R2 and R3 each represents hydrogen atom or an alkyl
group)
H C i\~ H
Il I
Rs - C C - R~ (3)
N
(wherein RG represents an alkyl group and R5 represents
hydrogen atom or an alkyl group)
R 7 ~ 1 ~C R G ( 4 )
(wherein R6 represents an alkyl group or an alkylthio group
and R7 represents hydrogen atom or an alkyl substituent at
the 4- or S-position)
2~ ~9462
R 8 ~ S
(wherein RB represents hydrogen atom or an alkyl group)
/\ S, I
Rg O C - S - C ~ (6)
~ C O O H
(wherein R9 and Rlo each represents hydrogen atom or an alkyl
group)
Rll
~C
11
HS'' ~ ~ - - SM (7)
/R12
(wherein Rl~ is either \R or -NHR~4, where Rl2, Rl3,
and R~4 each represents an alkyl group or an alkylene group,
and M represents an alkali metal (e.g., Na, K, Li) or
hydrogen atom).
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In the benzotriazole derivatives represented by
formula (1), R1 may be a hydrogen atom but higher tackiness is
generally obtained when R1 is an alkyl group having 8 or more
carbon atoms.
In the imidazole derivatives and benzoimidazole
derivatives represented by formulae (3) and (4), respectively,
R4 is preferably an alkyl group, R5 is preferably a hydrogen
atom or an alkyl group, R6 is preferably an alkyl or alkylthio
group having 8 or more carbon atoms and R7 is preferably a
hydrogen atom or an alkyl substituent at the 4 - or 5 -
position because such groups generally bring about higher
tackiness.
In the benzothiazole thiofatty acid derivatives
represented by formula (6), R1o preferably has 1 or 2 carbon
atoms.
Further, in the triazine derivatives represented by
formula (7), R12, R13, and Rl4 each preferably has 8 or more
carbon atoms.
At least one of the tackiness-imparting compounds is
dissolved in water. It is preferred that this solution
(hereafter referred to as a "a tacky layer-forming solution")
be used after being rendered acidic (i.e., pH < 7), more
preferably slightly acidic with a pH of about 3 to 4. In the
case where the exposed metal to be treated is copper, an
inorganic acid such as hydrochloric acid, sulfuric acid, nitric
acid, or phosphoric acid may be used for the purpose. Organic
acids such as formic acid, acetic acid, propionic
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acid, malic acid, oxalic acid, malonic acid, succinic acid,
and tartaric acid may also be used.
Although the concentration of the tackiness-imparting
compound in the solution is not strictly limited, it is
suitably regulated according to the solubility of the
compound and use conditions. The preferred range of the
concentration is from 0.05 to 20% by weight since such
solutions are easy to use. When the concentration is less
than 0.05 % by weight, formation of a thin tacky film tends
to be insufficient.
The treatment for imparting tackiness is conducted
either by applying the solution on the exposed metallic part
on which a solder film is to be formed or by immersing that
metallic part in the solution. In the treatment, a
temperature slightly higher than room temperature enables
formation of a tacky layer at a moderate rate in a large
amount. Although the temperature for the treatment is not
particularly limited because it varies depending on the
concentration of the tackiness-imparting compound, the kind
of the metal, etc., the preferred range thereof is generally
from 30 to 60~C. The immersion time is not particularly
limited, but it is preferable from the standpoint of working
efficiency that other conditions be controlled so as to
regulate the immersion time within the range of from 5
seconds to 5 minutes. In this case, it is preferred to use a
tacky layer-forming solution containing copper ions in an
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amount of 100 to 2,000 ppm (by weight), desirably 100 to
1,000 ppm, because use of this solution improves the
efficiency of tacky layer formation, i.e., the rate of the
layer formation, the amount of the layer, etc.
Before the printed circuit board is subjected to
treatment with the tacky layer-forming solution, only the
metallic circuit part on which a solder circuit is to be
formed is kept in an exposed state, with the other metallic
circuit part being covered with a resist or other material.
In the case where the whole of an electronic part is immersed
in the tacky layer-forming solution, the metallic part other
than the part on which a solder film is formed is covered
with a molding resin so as to avoid contact with the
solution.
Upon the immersion in or coating with the solution
containing the tackiness-imparting compound described above,
the compound adheres to the exposed metallic surface to
render the surface tacky. After water-washing and drying, a
printed circuit board or metallic part in which the exposed
metallic surface is tacky is obtained. A solder powder is
sprinkled over the tacky surface to allow the solder powder
to adhere thereto and the excess solder powder is removed.
Thereafter, the adherent solder powder is melted and leveled
by heating to thereby form a solder film. The thickness of
the solder film is preferably from 1 to 200 ~m while it
varies depending upon use conditions of the solder film.
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.
If the solder film thus formed has a thickness below
a required level, the thickness of the solder film
(especially on a metal other than copper) can be increased to
the desired level by conducting the treatment with the tacky
layer-forming solution two or more times, since the tacky
layer-forming solution imparts tackiness also to solder
surfaces though this tackiness is weaker than that imparted
to copper.
For adhering a powdered solder to the tacky surface,
various methods may be used. Examples thereof include a mask
method in which a solder powder is allowed to adhere through
a mask having an opening; a spray method in which a solder
powder is sprayed over the whole of an exposed metallic
surface having the tacky surface and the unnecessary solder
powder is then removed; a fluidized bed method in which the
printed board having the tacky surface is passed through a
tank in which a solder powder is suspended; and a roller
method in which a solder powder layer formed on a tacky
roller or mesh roller is brought into contact with the
printed board.
One example of simple methods for adhesion comprises
holding the printed circuit board having a tacky surface on
the exposed metallic surface on a slant, and dropping a
solder powder from above little by little. The appropriate
range of the slant angle of the exposed metallic surface is
from 10 to 85 degrees, preferably from 20 to 75 degrees, with
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the normal line. The slant angle below 10 degrees is
undesirable because the surface partly remains exposed with
no solder powder adherent thereto, while the slant angle
above 85 degrees is undesirable in that a solder powder is
unlikely to fall off.
It is preferable that the exposed metallic surface
having a tacky surface be vibrated in order to densely adhere
the solder powder. This technique is particularly effective
in forming a solder film on a through-hole part or on the
exposed metallic circuits of a double-sided printed circuit
board. In one embodiment of this technique, the exposed
metallic surface having a tacky surface is buried in a solder
powder and the printed board is transferred in a vibratory
conveyor.
The amplitude of the vibration for the adhesion of
solder powder may be from 0.1 to 20 mm. However, in the case
of transferring the printed circuit board by means of
vibration as shown in Fig. 2, the amplitude is preferably
from 1 to 10 mm. The frequency may be from 100 to 3,000 cpm
(cycle per minute), and preferably from 500 to 1,500 cpm.
The effective range of the vibrational angle (from the
horizontal) is from 0 to 90 degrees. In the case of
transferring the printed board by means of vibration as shown
in Fig. 2, the preferred range thereof is from 30 to 60
degrees.
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In the case where the printed circuit board has a
tackiness-imparted metallic circuit on one side thereof, a
solder powder is fed to the printed board with the circuit
side facing upward, and the printed board is vibrated to
densely adhere the solder powder to the tacky part without
forming a void or other defects. If it is desired to
continuously conduct the above method, this may be
accomplished as follows. The printed circuit board is placed
on a vibratory floor and the floor is moved, or the printed
board is placed on a slant floor and the floor is vibrated.
A solder powder is continuously fed to the tackiness-imparted
part of the printed board which is kept moving along with the
solder powder, thereby densely adhering the solder powder by
means of vibration.
In the case where the printed circuit board has a
tackiness-imparted metallic circuit in a through-hole part
thereof or on both sides thereof, the printed circuit board
is placed on a solder powder layer and the circuit board is
vibrated while a solder powder is being fed to the upper side
thereof. If it is desired to continuously conduct the above
method, this may be accomplished by placing the printed
circuit board on a solder powder layer formed on a slant
floor and a vibration is applied in a slant direction to
vibrate the solder layer.
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By thus bringing the printed circuit board into
contact with a solder powder layer with vibration, the solder
powder can be most densely adhered to the tacky surface.
Methods for practicing the above are explained below
with reference to the Figures.
In Figs. 1 and 2, numeral 1 denotes a printed circuit
board which has a through-hole part or to which tackiness has
been imparted at exposed metallic circuit parts on both sides
thereof. This printed board is placed on a solder powder
layer 3 on a vibrator 4, and is vibrated while a solder
powder 2 is being fed thereto from above, thereby densely
adhering the solder powder. The vibration may be in any of
the vertical direction, a horizontal direction (reciprocating
or circular motion), and a slanting vertical direction. When
the solder layer is to be moved in one direction, the whole
can be moved smoothly by keeping the floor oblique and
applying a vibration in a slanting vertical direction.
By conducting the treatment described above, solder
particles 14 can be densely adhered to a tacky layer 13
formed by the tackiness-imparting compound on the surface of
the through-hole part through which copper circuits 12 on
both sides of an insulating substrate 11 are connected, as
shown in Fig. 3.
The material of the solder powder can be arbitrarily
selected, according to use, from eutectic, silver-containing,
2 1 0 9 4 6 2
and bismuth-containing solders and other solders. The solder
powder preferably has a diameter of from 1 to 500 ~m.
It is preferred to cover the surface of the solder
powder with a resin composition containing a rosin or a rosin
derivative as an essential component and further containing
at least one of a carboxylic acid, an amine, an amine salt,
and a wax, because the thus-coated solder powder shows
improved reflow properties and in turn, improves the
reliability of electrical connection.
Examples of the rosin derivative that can be used as
an essential component of the resin composition include
hydrogenated rosins, disproportionated rosins, polymerized
rosins, rosins modified with maleic acid, aldehyde-modified
rosins, rosin esters, and rosin-modified phenolic resins.
The rosin or rosin derivative is an essential ingredient for
enabling the solder particle surfaces to impart a film-
forming ability. If the rosin amount is too small, these
effects are reduced. If a rosin is used alone, reflow of the
solder powder does not proceed smoothly.
For the purpose of improving reflow properties, at
least one of a carboxylic acid, an amine, an amine salt, and
a wax is added to the rosin according to one preferred
embodiment of the present invention.
The carboxylic acid, which has a carboxyl group, is
not limited in carbon number, etc., and examples thereof
include propionic acid, caprylic acid, lauryl acid, stearic
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acid, acetic acid, adipic acid, citric acid, malic acid,
maleic acid, and oxalic acid. The suitable amount of the
acid to be added is from 1 to 10% by weight. If it is more
than 10% by weight, the acid may cause corrosion of the
circuit conductor.
Examples of the amine or amine salt include
methylamine, ethylamine, butylamine, isopropylamine,
cyclohexylamine, monoethanolamine, diethanolamine,
triethanolamine, or a hydrochloric acid salt or bromic acid
salt thereof. The suitable amount of the amine or amine salt
to be added is from 0.1 to 5% by weight. If the amount
exceeds 5% by weight, an ionic residue remains in an
increased amount after reflow.
As the wax, carnauba wax, a synthetic wax, Japan wax,
or the like can be used. Generally in an amount of from 0.5
to 5% by weight. If the amount exceeds 5% by weight, the
adhesion effect is impaired.
These carboxylic acid, amine, amine salt, and wax may
be used alone or may be used in combination of two or more
thereof. By use of these, reflow properties can be improved,
ma~ing it possible to form a precise and fine pattern. It is
preferable that the resin composition be formulated so as to
have such properties as a softening point of 150~C or lower
and an acid value of 50 or higher. If the softening point
thereof is above 150~C or if the acid value thereof is below
50, smooth reflow may not be attained.
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2 ~ 0 9 4 6 2
A preferred example of the resin composition
comprises 89-97% by weight of the rosin component, 2-8% by
weight of the carboxylic acid, 0.1-1.5% by weight of the
amine or amine salt, and 0.5-1.5% by weight of the wax.
With the resin composition thus prepared, the surface
of solder powder is covered at a thickness of 0.05 to 10 ~m.
As a covering method, an ordinary technique such as spray
drying can be used. Too large amount of the resin
composition results in increase in the amount of ionic
residues, and thus is not prefered.
Use of a fine solder powder for forming a solder
circuit on a printed circuit board has a merit that a highly
precise and fine pattern can be formed, but has a problem
that the area of solder powder surface that is subject to
oxidation is large. In the case where a solder powder
containing a large amount of oxide is used, the adverse
effect of the oxide film can be eliminated by using a flux,
thereby to enhance the bonding strength of the solder.
However, when a flux is applied on extremely fine patterns of
solder powder, the solder patterns are apt to be damaged, so
that not only solder bridging tends to occur between fine
patterns in the solder-coated printed board obtained, but
also fluctuations in solder film thic~ness may be
experienced. In such a case, the solder particle pattern
adherent to the tacky surface may be fixed by heating. This
technique is preferred because it enables the application of
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a flux to be conducted without damaging the solder pattern.
Illustratively stated, after a solder powder is adhered to
the tacky surface, the adhered solder powder is heat-treated
at a temperature of 100 to 250~C for a period of 5 to 60
seconds, for example, at 160~C for 30 seconds or 180~C for 10
seconds, under such conditions as not to melt the solder.
Thereafter, a flux is applied, and the solder powder is then
melted and leveled to form a solder film.
In this case, even when the eutectic solder, which
has a melting point of 183~C, is heated to a temperature
above the melting point, the solder powder can be fixed
without being melted as long as the heating time is short,
probably because the surface of the solder powder is covered
with an oxide film. In general, a lower heat-fixing
temperature necessitates a longer heating time and a higher
heat-fixing temperature necessitates a shorter heating time;
it is necessary to select conditions that do not melt the
solder powder. If the heat-fixing temperature is below
100~C, a sufficient effect is difficult to obtain even when
the heating time is long. On the other hand, in the heat
treatment at above 250~C, oxidation of the surface of the
solder powder proceeds, causing a trouble in melting and
bonding. It is, therefore, desired to avoid prolonged heat
treatment at a high temperature. The flux to be used is not
particularly limited as long as it is suited to the solder
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powder used. Preferred is a chlorine-free flux such as a
rosin-based flux.
After application of a flux, the solder powder is
melted with, e.g., a reflow furnace, whereby a highly precise
and fine solder film which is free from a solder bridge and
has a uniform thickness can be formed on the exposed metallic
surface.
The present invention is based on a completely novel
technique developed for solder film formation on only a
predetermined part of a metallic surface, which technique
comprises imparting tackiness to the exposed metallic part in
which a solder circuit is to be formed and then adhering a
solder powder thereto to thereby form a precise and fine
solder film pattern.
According to the present invention, it has also
become possible to form a tacky substance on only an exposed
metallic surface by a chemical means in which the surface is
immersed in or coated with a tacky layer-forming solution
containing a tackiness-imparting compound and to adhere a
solder powder to the tacky surface to thereby form a solder
film.
Although the mechanism of the above-described
reaction has not been fully elucidated, it is thought that
the metal and the tackiness-imparting compound of the present
invention form a chelate compound showing tackiness.
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2 1 0 9 4 6 2
Since the thus formed tacky substance deposits on
only an exposed metallic surface (e.g., a pad of a solder
circuit), there is no need of conducting positioning for
tacky substance deposition and it has become possible to
sufficiently cope with the trend toward circuit pitch
reduction.
Furthermore, since a solder is adhered to this tacky
substance in the present method, the formation of a bridge
can be prevented by selecting an appropriate solder particle
size and, hence, a fine solder film pattern can be easily
formed, unlike the printing method which employs a solder
powder paste as an ink.
The present invention will be explained below in more
detail by reference to the following Examples, but the
invention is not construed as being limited thereto.
EXAMPLE 1
A 2% by weight aqueous solution of an imidazole
compound represented by formula (3), wherein the alkyl group
of R4 was C~lH23 and R5 was a hydrogen atom, was pH-adjusted to
about 4 with acetic acid, giving a tacky layer-forming
solution. This aqueous solution was heated to 40~C, and a
copper-clad, single-sided, flexible printed circuit board
which had been pretreated with an aqueous hydrochloric acid
solution and had a pitch of 0.3 mm was immersed in the tacky
layer-forming solution for 3 minutes to form a tacky
substance.
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The resulting printed circuit board was then washed
with water and dried. It was ascertained thereafter that the
tacky substance had deposited precisely on the pads only. An
eutectic solder powder having an average particle diameter of
about 40 ~m was sprinkled over the dry printed circuit board,
which was then lightly brushed to thereby adhere the solder
powder selectively to the tacky substance parts. Thereafter,
the solder powder was melted in a 240~C oven. As a result, a
thin eutectic solder layer having a thickness of about 20 ~m
was formed with high precision on the exposed copper circuit
parts.
EXAMPLE 2
The same procedures as in Example 1 were conducted
except that copper ions were added at a concentration of 200
ppm, the concentration of the tackiness-imparting compound
used in Example 1 was changed to 0.5% by weight, and the
immersion time was changed to 30 seconds. Despite the lower
imidazole compound concentration and the shorter immersion
time than those in Example 1, the formation of a tacky layer
was on a level satisfactory for solder particle adhesion, and
a thin eutectic solder layer having a thickness of about 20
~m and almost the same as that in Example 1 was formed with
.
hlgh preclslon.
EXAMPLE 3
The same procedures as in Example 1 were conducted
except that a 0.2% by weight aqueous solution of a
2 ~ O 9 4 6 2
benzoimidazole compound represented by formula (4), wherein
R6 was C8Hl7 and R7 was a hydrogen atom, was used, and that a
copper-clad, single-sided, flexible printed circuit board
which had been pretreated with an aqueous hydrochloric acid
solution and had a pitch of 0.25 mm was used. The solder
circuit thus obtained had sufficiently coped with the 0.25-mm
pitch.
EXAMPLE 4
The same procedures as in Example 3 were conducted
except that a benzoimidazole compound represented by formula
(4), wherein R6 was a butylthio group and R7 was a hydrogen
atom, was used. As a result, a highly precise and fine
solder circuit having a pitch of 0.25 mm was obtained as in
Example 3.
EXAMPLE 5
A tacky layer-forming solution was prepared by pH-
adjusting a 0.5% by weight aqireous solution of 5-lauryl-
benzotriazole (a compound of formula (1)) to about 3 in the
presence of sulfuric acid and methyl alcohol. This solution
was heated to 50~C, and the subsequent procedures were
conducted in the same manner as in Example 1. As a result, a
satisfactory circuit was obtained as in Example 1.
EXAMPLE 6
A tacky layer-forming solution was prepared by pH-
adjusting a 0.2% by weight aqueous solution of 5-butyl-2-
mercaptobenzothiazole (a compound of formula (5)) to about 4
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in the presence of methyl alcohol and triethanolamine. Using
this aqueous solution, the same procedures as in Example S
were conducted. As a result, a satisfactory circuit was
obtained ~s in Example 1.
EXAMPLE 7
A tacky layer-forming solution was prepared by pH-
adjusting a 0.5% by weight aqueous solution of 4-methyl-
naphthotriazole (a compound of formula (2)) to about 3 in the
presence of sulfuric acid, triethanolamine, and methanol.
Using this aqueous solution, the same procedures as in
Example 5 were conducted to obtain good results as in Example
1.
EXAMPLE 8
A tacky layer-forming solution was prepared by pH-
adjusting a 0.5% by weight aqueous solution of 2-[2-
(benzothiazolyl)thio]propionic acid (a compound of formula
(6)) to about 4 in the presence of methanol. Using this
aqueous solution, the same procedures as in Example S were
conducted to obtain good results as in Example 1.
EXAMPLE 9
A 0.5% aqueous solution of 6-diisooctylamino-1,3,5-
triazine-2,4-dithiol monosodium salt represented by formula
(7), wherein Rll was a diisooctylamino group (-N-iso-C8Hl7)
~iso-C8Hl7
and M was Na, was heated to 80~C to obtain a treating
solution for tacky layer formation. A printed board having a
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0.3 mm-pitch exposed copper circuit parts for connection with
a QFP (quad flat package) was immersed in the treating
solution for 10 minutes, pulled up, washed with water, and
then dried. As a result, only the exposed copper circuit
part surfaces had developed tackiness. A solder powder
having a diameter of 40 ~m was adhered to the tacky parts and
then melted with a reflow furnace regulated to have a
temperature of 240~C to thereby obtain a solder coat having
an almost uniform thickness of about 20 ~m.
EXAMPLE 10
A 0.5% aqueous solution of 6-oleylamino-1,3,5-
triazine-2,4-dithiol monosodium salt represented by formula
(7), wherein Rll was an oleylamino group (-NHC8H1~CH=CHC8Hl7)
and M was Na, was heated to 80~C, giving a treating solution.
The subsequent procedures were conducted in the same manner
as in Example 9. As a result, a satisfactory solder coat was
obtained.
EXAMPLE 11
A continuous sheet comprising a polyimide film
substrate and a copper foil laminated thereto was subjected
to etching by an ordinary method to obtain a copper pattern.
Thereafter, a polyimide film having punched-out openings
corresponding to the parts to be solder-coated was positioned
on and laminated to the copper pattern with an adhesive. The
continuous sheet thus obtained was subjected to a pretreat-
ment in which the exposed copper parts were lightly etched by
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an ordinary method. The pretreated sheet was then treated by
passing it, over a period of 30 seconds, through a 50~C bath
of a 1% by weight aqueous acetic acid solution of 2-
undecylimidazole that forms a chelate compound with copper
metal which solution had been pH-adjusted to about 4 with
acetic acid. The resulting sheet was washed with water and
dried to form a tacky layer of about 0.2 ~m thick selectively
on the exposed copper surfaces. Thereafter, a solder powder
having an average particle diameter of 40 ~m was adhered to
the tacky layer and melted in a reflow furnace. Thus,
printed circuit boards having a solder coat of about 20 ~m
thick were obtained as a continuous sheet.
In the case of patterns of the above-described type,
1,150 independent printed circuit boards were able to be
obtained by the method of the invention on the continuous
sheet substrate having an area corresponding to that
necessary for obtaining 1,000 printed circuit boards by the
conventional electroplating method in which dummy parts
should be formed for electrical connection according to
pattern arrangement. This shows that 15% extra printed
circuit boards can be produced, so that a production cost of
the printed circuit can be reduced.
EXAMPLE 12
A 2% by weight aqueous solution of an imidazole
compound represented by formula (3), wherein the alkyl group
of R4 was CllH23 and R5 was a hydrogen atom, was pH-adjusted to
2 1 0 9 4 6 2
about 4 with acetic acid, giving a tacky layer-forming
solution. This aqueous solution was heated to 40~C, and a
QFP which had been pretreated with an aqueous hydrochloric
acid solution and had a pitch of 0.3 mm was immersed therein
for 3 minutes to form a tacky substance.
The resulting QFP was then washed with water and
dried. It was ascertained thereafter that the tacky
substance had deposited precisely on the lead frames only.
An eutectic solder powder having an average particle diameter
of 40 ~m was sprinkled over the dry QFP, which was then
lightly brushed to thereby adhere the solder powder
selectively to the tacky substance parts. Thereafter, the
solder powder was melted in a 240~C oven. As a result, a
thin eutectic solder layer having a thickness of about 20 ~m
was formed with high precision on the lead frames.
EXAMPLE 13
The same procedures as in Example 12 were conducted
except that copper ions were added at a concentration of 200
ppm, the concentration of the tackiness-imparting compound
used in Example 12 was changed to 0.5% by weight, and the
immersion time was changed to 30 seconds. Despite the lower
imidazole compound concentration and the shorter immersion
time than those in Example 12, the formation of a tacky film
was on a level satisfactory for solder particle adhesion, and
a thin eutectic solder layer having a thickness of about 20
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~m and almost the same as that in Example 12 was formed with
high precision.
EXAMPLE 14
The same procedures as in Example 12 were conducted
except that a 0.2% by weight aqueous solution of the
benzoimidazole compound represented by formula (4), wherein
R6 was C8Hl7 and R7 was a hydrogen atom, was used, and that a
TAB (tape automated bonding) which had been pretreated with
an aqueous hydrochloric acid solution and had a pitch of 0.25
mm was used. The solder coat thus obtained had sufficiently
coped with the 0.25-mm pitch.
EXAMPLE 1S
The same procedures as in Example 14 were conducted
except for using a benzoimidazole compound represented by
formula (4), wherein R6 was a butylthio group and R7 was a
hydrogen atom. As a result, a highly precise and fine solder
coat having a pitch of 0.25 mm was obtained as in Example 14.
EXAMPLE 16
A tacky layer-forming solution was prepared by pH-
adjusting a 0.5% by weight aqueous solution of 5-lauryl-
benzotriazole (a compound of formula (1)) to about 3 in the
presence of sulfuric acid and methyl alcohol. This solution
was heated to 50~C, and the subsequent procedures were
conducted in the same manner as in Example 12. As a result,
a satisfactory solder coat was obtained as in Example 12.
EXAMPLE 17
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2 ~ 0 9 4 6 2
A tacky layer-forming solution was prepared by pH-
adjusting a 0.2% by weight aqueous solution of 5-butyl-2-
mercaptobenzothiazole (a compound of formula (5)) to about 4
in the presence of methyl alcohol and triethanolamine. Using
this aqueous solution, the same procedures as in Example 16
were conducted. As a result, a satisfactory solder coat was
obtained as in Example 12.
EXAMPLE 18
A tacky layer-forming solution was prepared by pH-
adjusting a 0.5% by weight aqueous soluiton of 4-methyl-
naphthotriazole (a compound of formula (2)) to about 3 in the
presence of sulfuric acid, triethanolamine, and methanol.
Using this aqueous solution, the same procedures as in
Example 16 were conducted to obtain good results as in
Example 12.
EXAMPLE 19
A tacky layer-forming solution was prepared by pH-
adjusting a 0.5% by weight aqueous solution of 2-[2-
(benzothiazolyl)thio]propionic acid (a compound of formula
(6)) to about 4 in the presence of methanol. Using this
solution, the same procedures as in Example 16 were conducted
to obtain good results as in Example 12.
EXAMPLE 20
A 2% by weight aqueous solution of an imidazole
compound represented by formula (3), wherein the alkyl group
of R4 was CllH23 and R5 was a hydrogen atom, was pH-adjusted to
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2 1 0 9 4 6 2
about 4 with acetic acid, giving a solder tacky layer-forming
solution. This aqueous solution was heated to 40~C, and a
copper-clad, double-sided printed board which had been
pretreated with an aqueous hydrochloric acid solution and had
through-hole parts and a pitch of 0.3 mm was immersed in the
tacky layer-forming solution for 3 minutes to form a tacky
substance. The resulting printed board was then washed with
water and dried. A powdered eutectic solder having a
particle diameter of about 40 ~m was adhered to the printed
board by the method shown in Fig. 1. As a result, the solder
powder was densely adhered simultaneously on both sides.
Further, the solder powder was densely adhered also to the
inner wall of each through-hole having a diameter of 0.3 mm,
as illustrated in Fig. 3.
EXAMPLE 21
Tackiness was imparted to a copper foil-clad, double-
sided printed board in the same manner as in Example 20. A
powdered eutectic solder of 40 ~m was then adhered by the
method shown in Fig. 2 under conditions of a frequency of 100
cpm, an amplitude of 4 mm, and a vibrational angle of 30
degrees, while the printed board was being moved in the same
direction as the powder. As a result, the solder powder was
densely adhered to both sides of the printed board. Further,
the solder powder was densely adhered also to the inner walls
of 0.3-mm through-holes.
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EXAMPLE 22
A resin composition was prepared by mixing 93% by
weight of rosin having a softening point of 100~C and an acid
value of 120 with 5% by weight of caprylic acid, 1% by weight
of cyclohexylamine bromate, and 1% by weight of carnauba wax.
The surface of eutectic solder powder having an average
particle diameter of 50 ~m was covered with this resin
composition by a spray drying method at a thickness of about
5 ~m.
Subsequently, a tacky layer-forming solution obtained
by pH-adjusting a 2% by weight aqueous solution of 2-
undecylimidazole to about 4 with acetic acid was heated to
40~C. A copper foil-clad, single-sided, flexible printed
board which had been pretreated with an aqueous hydrochloric
acid solution and had a pitch of 0.3 mm was immersed in the
tacky layer-forming solution for 3 minutes to form a tacky
substance.
The resulting printed circuit board was then washed
with water and dried. Thus, the tacky substance was
deposited precisely on pads only. The coated eutectic solder
powder obtained above was then sprinkled over the dry printed
circuit board, and the excess solder powder was removed by
lightly brushing the circuit board to thereby attain
selective adhesion to the tacky parts. Thus, a satisfactory
solder powder pattern was obtained.
2 1 0 9 4 6 2
The circuit board having this solder powder pattern
thereon was heated in a 250~C conveyor oven to melt and level
the solder powder, thereby giving a satisfactory solder
circuit pattern.
This solder circuit pattern thus obtained had a
uniform solder film thickness of 30 ~m. The concentration of
residual ions was 8 ~g NaCl/ inch2 as determined without
washing.
EXAMPLE 23
A printed circuit board which was for connection with
a 0.25 mm-pitch TAB and in which copper in the corresponding
0.25 mm-pitch copper pads had been exposed, was immersed at
40~C for 30 seconds in a 1~ by weight aqueous solution of 2-
dodecylimidazole whose pH had been adjusted to about 4 with
acetic acid. The resulting printed board was washed with
water and dried to selectively form a tacky substance layer
on the surfaces of the copper pads. An eutectic solder
powder having an average particle diameter of about 50 ~m was
sprinkled over the printed circuit board, which was then
lightly brushed thereby to obtain a highly precise and fine
solder pattern in which the solder powder was selectively
adherent to the tacky substance parts.
The resulting printed circuit board was heated at
170~C for 30 seconds to fix the solder powder. Subsequently,
a commercially available, rosin-based flux was applied and
the printed board was then placed in a 230~C reflow furnace
- 2 '~ 0 9 4 6 2
for 1 minute to thereby melt the solder powder and form a
solder circuit. Thus, a printed circuit board having the
copper pad surfaces covered with a highly precise and fine
solder coat having a uniform thickness of 30 ~m was obtained.
Printed boards thus obtained were randomly sampled to examine
50 samples for solder bridging. As a result, all the samples
had no problem.
EXAMPLE 24
The same procedure as in Example 20 were conducted,
except that the circuit board was thrusted vertically into a
solder powder layer which was being shaked by a vibrator
under conditions of a frequency of 3,000 cpm, an amplitude of
1 mm, as shown in Fig. 4 wherein numerals 1, 3 and 4 denote a
printed circuit board, a solder powder layer and a vibrator,
respectively. As a result, the solder powder was densely
adhered on both sides and inner walls of the printed board.
The solder film-forming method according to the
present invention is based on a principle utterly different
from those in the conventional methods for solder film
formation. That is, the method of this invention comprises
simple procedures consisting only of immersing or coating the
exposed metallic circuit part(s) of a printed circuit board
in or with a tacky layer-forming solution containing a
specific tackiness-imparting compound to thereby impart
tackiness to the exposed metallic circuit part(s),
subsequently adhering a solder powder to the solder circuit
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2 ~ O 9 4 6 2
part(s), and then melting and leveling the solder powder.
Thus, it has become possible to accurately form a precise and
fine pattern without the necessity of troublesome operations
such as positioning.
Furthermore, since the solder film thus formed has no
bridges, products can be obtained at a high production
efficiency and a low off-specification rate.
While the invention has been described in detail and
with reference to specific embodiments thereof, it will be
apparent to one skilled in the art that various changes and
modifications can be made therein without departing from the
spirit and scope thereof.
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